In copending application Ser. No. 39,372, filed May 15, 1979, now abandoned I describe a printer which directs a plurality of small projectiles such as balls of about one millimeter in diameter in extremely rapid succession against a printing medium such as a ribbon overlying a sheet of paper. In the gun of the printer, which is movable about orthogonal axes for aiming, balls are introduced successively into a resilient breech which is slightly smaller in diameter than the balls. Air is maintained under superatmospheric pressure in a chamber behind the breech. The ball is fired by pushing it sufficiently far through the breech to snap it through to the barrel side and allow the pressurized air to expand into the barrel and propel the ball outwardly.
While the barrel of the aforementioned ball gun permits the air from the pressure chamber to act on the ball being propelled with a high degree of efficiency, it also has certain drawbacks. First, it has been found that deposits of dirt or other forms of contamination tend to build up in the area between the front edge of the construction and the rear edge of the barrel. If these deposits build up asymmetrically around the barrel, they can severely degrade or destroy the accuracy of the ball gun.
Secondly, the barrel must be accurately centered relative to the breech. In apparatus of the type contemplated, in which the ball diameter, and hence the barrel diameter, is less than a millimeter, achievement of the necessary tolerances can be quite difficult and expensive.